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Introduction

This post is a step-by-step tutorial for installation of the IndivisionAGA MKII hardware into an Amiga 1200 Desktop machine. The target Amiga already has a FastATA fitted and there are some extra (easy) steps that will be necessary in order to allow these two peripherals to be fitted together in the machine at the same time. This tutorial will also cover these steps.

My Amiga 1200 Desktop

What you will need

In order to complete a working installation of the IndivisionAGA MKII you will require the following:

*NOTE 1: These items are only required if you intend to install the IndivisionAGA with a FastATA.

Preparations

Step 1: Remove A1200 top enclosure

In order to complete this procedure you are going to need to take the cover off your Amiga. If you are unfamiliar with this kind of task then I can offer you the following advice:

Amiga computers were born to be modified. You are almost committing a crime if you don’t modify it. Modifying your Amiga comes with the territory, and should form part of the enjoyment you get out of using the machine. As long as you’re able to be careful, and take things slowly (thinking each stage through as you go along) you shouldn’t encounter any major difficulties.
That said, I do feel obligated to point out that damage to your Amiga is possible if you mess something up. If you’re already convinced that hardware modification is not your strong point then I would advise you to seek the assistance of a trained technician. AmigaKit is one company who can offer these kinds of services, and I would highly recommend them to you.

Step 2: Prepare work surface

First, prepare your work top. My Amiga is a Commodore edition in pristine condition so I like to avoid scratches and scuffs. If you feel the same then you’re going to want to put something soft down on your work top to prevent cosmetic damage to your machine when you work on it. The ideal solution would be an anti-static bench mat, but if you don’t have one of these (I didn’t either) then any soft material that does not tend to generate any static should work. I used an old towel.
Position your Amiga as shown below and then release all of the enclosure screws.

Release screws

Once you have released all of the securing screws, carefully turn your Amiga back over and then remove the top cover. Be careful because it is still tethered to the Amiga motherboard via the power/hard-drive LED wiring. You should be able to flip the top cover over as shown below:

Remove the top cover

Step 3: Remove the keyboard

The next step is to remove the keyboard. The keyboard is connected to the Amiga motherboard via a green flat flex-foil cable. Care must be taken to ensure that the mating connector clasp is released before you try to pull this flat cable out. In order to release the clasp, pull the top collar upwards on each side until it releases its hold on the flat cable.

Note that the clasp does not fully remove. It releases its hold on the flat cable and then sits in a released position on the connector housing. Do not try to remove the clasp from its housing because you will break it!

When the clasp is released it’ll look like the photo below:

Clasp Released

Once the clasp is released the flat cable should pull free without resistance. If you feel any resistance then you have not fully released the clasp.
With the cable pulled free you can lift the entire keyboard from its retaining points and set it to one side.

Keyboard Removed

Step4: Remove FastATA

If you have a FastATA fitted then you will need to remove it in order to allow fitting of the IndivisionAGA. If you don’t have a FastATA then you can skip this stage.

TIP
In my experience the FastATA Gayle connection is not to be tampered with. Once it has been connected, and it has settled in place, then the less it is disturbed the better. For this reason I would recommend leaving the Gayle connection alone – you can separate it from the FastATA motherboard by removing the IDC connector. This will leave you with just the FastATA motherboard to remove from the A1200 ROM sockets.

You need to take great care when removing the FastATA from the A1200 ROM sockets. The risks, if you’re not careful, are as follows:

Bending or snapping pins on the FastATA if it is released suddenly at an angle.

Damaging the FastATA PCB if it is levered against without care.

Damaging the A1200 motherboard if a sharp instrument is used to lever the FastATA from the ROM sockets.

FastATA removed

With these potential hazards in mind, use a flat instrument (preferably plastic but I was able to use a flat screwdriver with great care) to gently prise the FastATA from the ROM sockets. Work on all four corners of the FastATA a little bit at a time, exercising patience. The retention force of the ROM sockets will initially be very strong because the FastATA will have settled in its connection. Eventually, after some perseverance and patience, you will be able to release this connection and pull the FastATA free.

Step5: Removing the floppy drive

Remove Floppy Drive

In order to route the IndivisionAGA graphics cable it is better to remove the internal floppy drive. This item is easily removed by releasing its retaining screws and disconnecting the power and data cables. Please exercise care when removing the data cable as if it is pulled away at an angle you will bend pins.

Installing the hardware

Step 1: Fitting the IndivisionAGA

The IndivisionAGA main board connects over the top of the Lisa chip. Make sure you connect it with the orientation shown in the photo below!
When making the connection, press with equal force on the top of the PLCC socket so that it engages Lisa with the minimum angle possible. It will be necessary to press quite hard in order to complete the connection fully.

Don’t use any tools to force the IndivisionAGA onto Lisa! Use only your hands.

Connecting the Indivision

After you’ve fitted the Indivision to Lisa then you will need to route the graphics cable. Methods are varied but there is unfortunately no ‘perfect’ solution for this as of yet. My preferred ‘solution’ was to route the cable underneath the floppy drive (hence why I asked you to remove it) and then out of the rear expansion slot next to the mouse port. I left the DVI connector dangling out of this small access panel, but I secured it later by using the monitor cable fasteners. Not an ideal solution by any means, but it works.

Step 2: Fitting the ‘riser’ ROM sockets

40 way DIL socket (round pin)

If you are installing the Indivision with a FastATA, or you plan to use a FastATA in the future, then you will need to fit ‘riser’ sockets into the ROM locations so that the FastATA board will fit over the top of the Indivision. To do this you will need to fit one pair of 40-way DIL sockets into the existing ROM sockets. The best type of DIL sockets to buy are the ’round pin’ type as shown in my photo. You should be able to obtain these from AmigaKit, or you can get them direct from an electronics supplier such as Farnell, e.g. order code 1103855. Note that the ROM sockets fitted into the A1200 motherboard are 42-pin

Riser sockets fitted

but the front two pins are not used. Therefore you will need to fit your riser sockets so that they sit flush with the back of the A1200 motherboard sockets as shown in the following photo. Note that some Amiga users report that they prefer to fit two pairs of riser sockets so that the FastATA is hoisted up further, providing even more clearance. If you order four 40-way DIL sockets then you are free to experiment with this but my experience is that one set of riser sockets provides just enough clearance whilst also ensuring that the Fast ATA does not foul against the keyboard.

Step 3: Preparing the FastATA (insulating) for re-fitting

The FastATA motherboard is mostly through-hole construction, which means that the component connections protrude through to the bottom side of the board. These protrusions are quite lengthy and as such there is a risk of short-circuit to the Indivision PCB. In order to guard against short-circuits it is important to insulate the two boards in some acceptable manner.

FastATA Insulation Plate

In my experience I was able to find a piece of strong, thin plastic from an old ring-binder that I trimmed to size. I then fitted it to the bottom-side of the FastATA using two spots of hot-melt glue. Hot-melt glue is a good solution because it is easily removed at a later time should the need arise. Remember that the glue is not there to hold the FastATA board together – it only needs to keep your piece of insulation in place!
I think it goes without saying that the chosen insulation material must be non-conductive. Plastic is a good choice for this reason.

Step 4: Refitting the FastATA

Once you have settled on a suitable method of insulating the bottom-side of the FastATA from the Indivision you are ready to fit the FastATA back into the ROM socket risers. Be very careful to ensure that all the pins on the FastATA line up with your riser socket receptacles. Your FastATA will now entirely fill the ROM sockets because you fitted 40-way sockets into the existing 42-way sockets that are fitted to the A1200 motherboard. Once you have fitted the FastATA back into the ROM sockets you need to press firmly all around it to ensure a solid connection. If you end up with a dodgy ROM connection then you will experience all sorts of problems reading/writing to your storage devices so the connection needs to be very secure.

Step 4a: Tie down the Fast ATA?

Some Amiga users prefer to tie-wrap their FastATA motherboards in place to secure it from coming loose after fitting. I have had mixed experiences with this method and I have not found it to be necessary in a desktop machine, provided of course that your machine is not subjected to transportation. A modified desktop A1200 is rarely a portable one.
The problem I’ve found with tie-wrapping the FastATA is that it doesn’t quite hold the board down in a manner that is guaranteed to prevent the board from working loose. In fact, I have had experience of the tie-wraps themselves forcing the FastATA to retreat from its ROM sockets.

The ideal solution to this common problem would be to do some surgery on the A1200 motherboard, removing the existing ‘flat’ type ROM sockets and fitting the ’round’ type (like the ones you’ve used as risers) instead. This would provide a much stronger connection both electrically and mechanically. Unfortunately this is not a task for the amateur Amiga tinkerer because it can be very difficult to remove through-hole components from a multi-layer PCB like the A1200 motherboard. Still, it is possible and a good technician would be able to do it for you if you think it’s necessary.

In my case I found that I was able to complete this work without replacing the A1200 ROM sockets and without tie-wrapping the FastATA in place.

Note that you should strictly limit the number of mating cycles you subject the standard A1200 ROM sockets to. In practice this means that any time you dismantle your A1200 in the future you should always disconnect the FastATA from the riser sockets, leaving the riser sockets fitted to the A1200 motherboard. This is because your riser sockets are round-pin and the standard A1200 motherboard sockets are square-pin. A consequence of fitting a round peg into a square hole is that the connection quality is reduced, and the number of reliable mating cycles is once or twice at maximum before the connection will cease to be mechanically or electrically sound. If you compromise the standard A1200 ROM socket connection quality then you may be forced to have the ROM sockets replaced (if so use round pin types as described earlier!) so the best advice I can give you is connect the riser sockets once, then never touch them again.

Step 5: Refitting the floppy drive and keyboard

We’re almost done! The floppy drive goes back in the same way that it came out. Be careful when re-fitting the data cable because it’s all too-easy to miss a set of pins if you are complacent.
Once the floppy drive is re-fitted you can re-fit the keyboard. Make sure that the mating connector clasp collar is released before you try re-connecting the flex-foil. If the collar is released then there should not be any resistance when you try to mate the flex-foil to the connector. Grap the flex-foil with one hand so that it is held in a fully-home position inside the mating connector, and then with your other hand press the clasp collar back down into place.

Step 6: Testing your installation

Murphy’s law says that if you put the cover back on before testing your installation then your work is guaranteed to be a failure. For some reason as yet unexplained by science, you have to humour Murphy by testing it before putting the covers back on. If you don’t believe me, try fitting the cover back first and you’ll see what I mean!

First Test

In any case, testing first is a good idea. If there is any unexpected behaviour or problems then you are in a much better position to see what’s going on, and react appropriately, if you have the covers off.
If, for example, you have unwittingly created a short-circuit condition of some kind then you will often see, hear or smell evidence of this if your senses are able to be trained on the gubbins of your equipment. Reacting to these kinds of issues quickly will save your Amiga. Blissful ignorance of them (like when the cover is fitted) could damage your equipment beyond repair. Please take my advice!
If all goes to plan then you should see an Individual Computers logo appear on your monitor within 2-3 seconds of switching on your Amiga. If, after this, your Amiga boots into Workbench then you will also have confirmed that your FastATA is still working. If you don’t see the logo, or your Amiga doesn’t boot (or both!) then you will almost certainly have some kind of connection problem. Go back through your work, checking and re-checking all of the connections and making certain that all the socketted connections are seated firmly in place.

Step 7: Closing

After all is confirmed working Murphy will let you fit the covers back on and all will be well.

I hope you have found this tutorial to be of use. Please feel free to write a comment. Constructive criticism is also very welcome – I am an electronics engineer by trade but I certainly do not pretend to know it all!

If you have problems with your installation I may be able to offer some advice. In that case I would prefer you to comment so that the advice is then available to others, but I also welcome email – please see the ‘contact’ section of my website.

Introduction

The StorCenter ix2-200 is a RAID network drive supplied by iomega. I have used the 2TB version for about two years now to keep secure (backed up) copies of my precious data. Any data I write to the device is mirrored on its paired 2TB drive inside the unit, so one drive can fail and I’ll still keep my data.

Recently I’ve had cause for complaint with this unit’s default network setup routine. When you switch the device on it goes through a boot-routine which involves setting up the network address and subnet. If possible it does this via dhcp so if you’ve got it connected to your router it’ll be assigned an appropriate IP and will be instantly visible on the network.
The problems start when, for whatever reason, the device is not able to obtain network settings via dhcp. In that case it assigns itself an address in the range 169.254.x.x with subnet 255.255.0.0. In that case the network drive could end up with one of 65536 possible IP addresses in that range. How is one supposed to know what IP address it’s assigned itself?

Hardware Hacking

Location of JP1 on ix2-200

I had two choices. Set my computer to scan all of the 65536 possible IP addresses until it finds an active one. Or, take the unit apart and see what hardware hacking can be done. The former is probably quicker, but the latter is more fun. Hence, this hardware hacking blog was born.

With the unit apart, I found a conspicuous looking pin header called JP1. A few pokes around with my ‘scope revealed what looked like microprocessor level (3.3V) RS232 comms on one of the pins.

Completing the hack…

RS232 data on pin (2)

The next task was to try and see if I could view these signals on a PC. The main problem here is the fact that the data output is 3.3V logic levels (basically it’s the raw output from a microprocessor) and the RS232 input to a PC is +/-12V standard RS232 logic levels. It’s easily solved though, you just need to get yourself an RS232 level-shifter chip such as a MAX3232 and rig-up a circuit as per my schematic shown below, and then connect it to JP1 (as shown on the schematic) according to the pinout in the photo.

I only had an SMT version of the MAX3232 part in my junk bin so I soldered it onto some proto-board with the 0.1uF capacitors tacked on top and then I wired it up to JP1 as shown in the photo below.

My hacked on level shifter IC to interface with the PC

Viewing the data on the PC.

In order to view the data on a PC you simply need to put everything back together, connect the ends of your cables to a DB9 connector as shown in the schematic, and then connect the DB9 connector to your PC’s serial port via a standard 9-way serial cable. Then fire up a terminal (I recommend PuTTY) and enter the following settings:

PuTTY Terminal Settings

Once you’ve entered in the settings, select connect, and power on the NAS. If all goes well some boot-time debug data should start spitting out on the terminal. Something like that shown below:

Debug data coming through over the terminal

After 2-3 minutes you should be presented with a login prompt. If you want to gain root access to the NAS over your PC terminal simply log in with the following credentials:

USER: root
PASS: soho

Gaining root access via the terminal

Sweet!

That’s it – you’re in with root privileges. You can now enter the standard Linux commands and change whatever you wish. My main reason for going to all this trouble (apart from enjoying hardware hacking) was to find out the boot-time network settings it was assigning itself. Once I knew those I was able to gain access via the standard PC based web interface and change the settings to suit my home network.

I hope you enjoyed! Here is a quick video of the entire boot process and logging in:

A good find…

It’s a chore to find yourself lugging junk down to the tip. But every now and then you happen across a diamond in the rough! Today was my day to score as I spotted a Commodore CDTV unit lurking in a dark corner of the electrical disposal area. She was looking poorly and in desperate need of rescue. Needless to say I took on the challenge, plucking her from the ruin (yoink!) and saving her from certain destruction. I am now the proud owner of a CDTV unit!

Does she work?

Well… no, she didn’t. Completely dead. Hence the throw-away, I presume. But no self-respecting nerd would give up this easily; I released the top cover and had a bit of a poke around. The fault was actually quite easy to find – a 5A fuse on the secondary side of the PSU was blown. Of course, a blown fuse is rarely the root cause of the problem – usually some other fault is to blame. In this case, though, I replaced the fuse and Bob’s your uncle – she powered straight up!

Where is the fancy CD boot screen?

That was my first observation, too. CDTV units had a cool CD boot screen with a spinning disc and a laser. This unit was just booting to a kickstart 1.3 floppy screen. How disappointing!

JP15 Link

I noticed that the unit had a clunky old switch hanging off the rear. This was clearly a poorly implemented modification by a previous owner. An obvious application for a user-installed switch would be some kind of ROM-switcher, so I began to wonder if this was the case here. The switch terminated at a 3-pin molex, but it wasn’t connected anywhere and nor did there appear to be an obvious place for it to go on the motherboard. Eventually, though – after quite a long period hunting around the motherboard and scratching my head – I found JP15 which is an in-line 3 pin header. It wasn’t a molex receptacle but the pitch looked about right. Tried it on for size and it was a perfect fit!

CD boot screen

A couple of power cycles with each position of the clunky switch finally revealed the classic CD boot screen. Even by today’s standard it looks uber-cool. This photo doesn’t really do it justice:

HURAHH!

So, I am now a very proud owner of a Commodore CDTV. She’s not the best of examples (the reset button on the front is broken and the keyboard is a bit worse for wear) but she’s fully functional and I reckon she’ll make a nice base unit for some hardware hackery. It’d be nice to bring some modern computer peripherals to this machine and see if I can get them working together with her. A hard drive would be nice, for example. Hmmmm! To be continued!

Happy Pi Approximation Day 2013!

If you don’t know what the heck I’m talking about, then allow me to explain. In mathematics, science & engineering, π is regularly used in descriptions of circular motion. This includes electrical waveforms and A.C. theory. Today is the 22/7, and mathematicians commonly use the fraction 22/7 as an approximation of π. Put it into your calculator and you’ll get roughly 3.14.
The true (exact) value of π is unknown – it cannot be expressed as an exact decimal number. The key here is ‘decimal’ number, because after all our decimal number system that we choose to use is actually arbitrary! Computers have had a fair crack at it and they’ve managed to compute π to thousands of millions of decimal places using various types of algorithms! But, ultimately, it’s not possible to express it as an exact number.

π Approximation Day, held each year on 22/7, is an opportunity for number nerds all over the world to celebrate their favourite irrational number.

Behold the Pi Pie!

The traditional way to celebrate π approximation day is to bake a π pie. That’s my kind of celebration, so this year Nerys John and I collaborated on a batch of scrumptious mixed berry pi pies. Here’s how we did it:

What you’ll need

What you’ll need

Ingredients:

A roll of shortcrust pastry (amount depends on how many pies you want to make)

A package of fruits – we chose mixed berries (Tesco Summer Fruits)

Apples (you can add apples or not – your choice!)

A jar of granulated sweetener

Flour

A small amount of milk (for brushing onto the tops of the pies for browning)

A small jar of cinnamon

Tools:

A rolling pin

A light brush

A small pie baking tray

Small foil pie cases

A large dish

A sharp knife

A large pie cutting stencil (for making the pie bottoms)

A small pie cutting stencil (for making the pie tops)

A suitable π stencil. We used a home-made π stencil that we cut out of cardboard.

First, the berries

Pouring the filler

The first thing we need to do is pour the berries into a mixing dish. If you’re adding apple, you’ll also want to slice the apple up into small pieces and add them into the dish as well.

Add the cinnamon – with flippancy

Once you’ve added your chosen filling to the mixing dish you need to sprinkle some cinnamon onto the top of them and mix. Personally I’d like to be able to offer you exact amounts here, because that’s how I like to work, but apparently when it comes to baking it’s better to be flippant with your measurements – much to my dismay. Add an arbitrary amount of cinnamon such that your bowl resembles that of the photo on your right, and then mix.

Sugar coated! Mmmm

Next up, it’s time for my favourite bit – the sugar. Again, the Engineer in me wants to give you specific measurements here but unfortunately a good cook has to be skilled in flippancy so you’ll need to throw an arbitrary amount of the sweetener onto your mixed berries such that they vaguely resemble the photo on your left. If you’re thinking that baking is a lot like being tortured right now, then you’re not alone. My solution was to find someone experienced in flippancy to cast the ingredients – I recommend you do the same. Somehow certain humans have managed to build a resistance to this kind of chaos (shrug).

Mixed Berry Filling

Right then, now it’s time to mix everything together. Be careful here; you want everything to be nicely mixed, but you don’t want to pummel the berries into oblivion. If you do that you’ll end up with a soppy, watery mix and apparently that’s not good for the final result. The best way to do this is to find a nice large spoon and just turn the mix over and over gently until the ingredients blend together nicely.

Once you’ve mixed the berries and the flavouring you need to heat the filling up in the microwave for a short while. How long? Well, it’s a chef’s measurement again I’m afraid. Just try them in for a minute or so, stir, then try again until eventually you’ve got a pretty warm and well mixed filling. It’s normal for the filling to go a bit more watery during this process – the point is for the filling to become a bit more gooey (all fruit pies are gooey, right?) but not too gooey that the whole thing turns to water. You still want the filling to be stodgy with recognisable berries in the mix.

Now the pastry.

The rolled pastry

Cover your work surface with a dusting of flour so that the pastry can be moved around on the work top without binding and sticking. I’ve always been taught to use WD40 in this kind of scenario, but apparently that’s a bad idea here so I guess we just stick to the flour. Roll the pastry out to a flat sheet as shown in the photo on your left, whilst being mindful that the thickness of the rolled pastry will be proportional to the thickness of your pie casing. You want it thin, but not too thin. If in doubt then use the force, Luke. Let go.
Once the pastry is rolled it’s time to cut out your pie tops, pie bottoms, and π symbols. Using the larger pie stencil for the bottoms and the smaller pie stencil for the tops, cut as many pie bottoms and tops as you can afford to make whilst at the same time allowing enough room to make your π symbols. We’ll worry about the pie symbols in a moment, but for now just cut out your pie tops and pie bottoms, then peel them away from the pastry and put them to one side.

The Pi Symbols

Making Pi Symbols!

It would be really cool if we had a π shaped pastry cutter, like the ones you’ve used to cut the pie tops and bottoms. Unfortunately I didn’t have anything of the sort. I considered ordering some thin sheet metal (maybe copper) and making my own π-shaped pastry cutter, but perhaps that’s an improvement for next year. For this year we simply drew a π shape onto cardboard (crap artist? Me too – get someone

Don’t cut your fingers!

skilled to do it), then we cut out the card board shape, and used it as a stencil from which to cut around the pastry with a sharp knife. Something like that shown on your right. Depending on the amount of pies you’re intending on making this part can be a bit laborious and that’s where a decent home made metal (cutting) π stencil would come in handy. But persevere and the end result will be worth it 🙂

Preparing the bottoms, the filling, and the tops

Preparing the pie bottoms

Nearly there – it’s time to prepare your pies for baking. Carefully press your pie bottoms into the pie foils as shown in the photo on your left. You want a snug fit but don’t force the pastry too much or you’ll never get them out of the foil in one piece afterwards.

A filled pie!

Once the bottoms are in, it’s filling time. Spoon a generous amount of filling into the pie bottom (drain off any watery content beforehand) but be aware that you need to be able to comfortably fit the pie top on afterwards. If you fill them up too much then the filling will breach your pie tops a little bit when they’re in the oven. In my experience some of them breached and some of them didn’t. That’s what happens when you’re flippant, I reckon. No consistency. But good luck trying to persuade a chef to optimise their process! I think Engineers are banned from most kitchens.

Press fitted pi pies!

Finally you’ll need to press-fit the pie tops. Just drape the pie top over the filling such that it’s centralised, and then pinch all around the edges of the pastry where top meets bottom (if top

Brushing milk so that the pie browns.

doesn’t meet bottom, you’ve screwed up) so that the finished pie is sealed all the way around. Once you’re happy with the sealing you can place your π symbol on the top (you don’t need to press it on, it’ll adhere during the baking process) and then you just need to brush a small amount of milk over the pie for browning.

Bake!

The hard bit is over, but this is perhaps the most crucial part of the job in terms of getting the best possible result. Set your oven to 180 degrees centigrade (Fahrenheit? Pah – use proper units) and make sure it’s nice and hot before you put the pies inside. Once the oven has thoroughly warmed through then you can put your finished pies in for baking. It’s tempting to leave them in until they are nice and brown (like you do with a meat pie, for example) but in the case of fruit pies it’s better to watch them carefully and bring them out when they’re just lightly browned. If you leave them in too long the filling gets really hot and then you’ll end up with a breach of the warp core. It’s all over after that, and not even Geordie La Forge will be able to help you. So basically just be careful.

Finished!

And here’s the finished result. They were bloody lovely too! Thanks to Nerys John for directing the kitchen process. I could never have been this skilled on my own, and that would have meant that I’d never have managed a decent result. In some rare cases, Engineering isn’t the answer. This might be the only time I ever publically admit that!

Those of us who are fortunate enough to have been born in a developed country are blessed with learning opportunities that no society on Earth has ever enjoyed before. The information age has made it possible for rational human beings all over the developed world to discover and learn about any subject of their choosing. The amount that we learn, and what we do with that information, is limited by just two things:

Our imagination.

Our will to persevere.

Blessed as we are with this web of information and opportunity for learning, it is easy for most of us to ridicule the ignorance (I would rather call it stupidity) of the Flat Earth Society who, in spite of all the available information and evidence, still choose to peddle the ridiculous idea that the Earth is in fact flat.

So, we can set ourselves up on a pedestal, right? We’re better than those flat earth ignoramuses because we take pride in our learning opportunities and we delight in the power of evidence based discovery.

Well… not quite. It’s surprising how many other elementary science based misconceptions are in regular use among our rational society. People like you and me; people you work with, go to school with, your neighbours, your close friends, your spouses and children – any of these can potentially be guilty of them. They seem to slip under the radar in terms of ridicule, but I think some of them give the flat earthers a run for their money. In honour of Super Moon Sunday 2013 , try these common Moon based misconceptions on for size. Some of them will make you cringe!

Unfortunately my attempts were rather less successful! I was met with the dreaded blinking power LED and a continuous cycle of repeated resets. Drat. What could possibly have gone wrong after a mere quarter of a century?

A quick look under the cover revealed the very obvious culprit:

Amiga A500+ Battery Leakage

More Amiga A500+ Battery Leakage

Also, rather bizarrely, the battery fuelled corrosion seems to have spread all the way over to poor old FAT AGNUS!

FAT AGNUS battery leakage

It seems like this may be terminal for my trusty old Amiga 🙁 but I will break out the old toothbrush over the weekend and see if I can clean her up a bit. With a bit of luck a good clean will see her up and running again. Wish me luck!

Introduction

The Sinclair ZX range of computers were among the first affordable computers and kick-started the UK’s home computer revolution in the 1980s. These machines relied on consumer television equipment for their display output and, as such, they featured an RF modulated output so that users could simply “tune-in” to them on their television sets.

Modern TVs use digital tuners and thus are no longer compatible with the analogue output of the Sinclair ZX computers. However, most TVs – even the most modern models – still feature composite video inputs and it turns out that it’s very easy indeed to modify the video output on these computers to be compatible with a TV composite video input. As well as providing compatibility with modern television sets, this modification also improves the computer’s video output quality. This post will detail the procedure for modification of the Sinclair ZX Spectrum Computer, but it should be fairly similar on other Sinclair models of that era.

As shown in the basic block-diagram below, the basic principle of video generation and display for the ZX Spectrum Computer is as follows:

Generate composite video output from memory map

Add chrominance (colour)

Generate UHF with RF modulator

Send down cable to TV

Recover composite video with TV UHF demodulator

Display

Video display block diagram

As can be seen, the modulation/demodulation process is actually quite wasteful because we start with composite video, modulate it at the Spectrum video output, and then we just de-modulate it at the TV end to get composite video again! This was convenient at the time the computer was designed because it allowed users to display the computer output on standard analogue tuner TVs, but with most modern TVs featuring composite video inputs it’s pointless – we can just cut the mod/de-mod process out and connect standard composite video out straight from the Spectrum Computer directly to the TV composite inputs!

Completing the modification

The modification is very straightforward. All you need to do is disconnect and remove the RF modulator, and then connect the machine’s composite video compatible output directly to the RCA connector via a decoupling capacitor. Here is the process broken down into stages:

1: Remove the front cover

Cover Removed

To remove the front cover, release the 5 screws on the bottom of the machine. The front cover will then lift away, but BE CAREFUL of the keyboard flat cables – you will need to disconnect these before you can lift the front cover all the way off. They can be a little bit tight but with care they will simply pull out. After this you should be able to remove the front cover completely. The item we are going to remove is the RF Modulator unit which is enclosed inside a metal screening can at the top left-hand side of the unit, shown in the photo to the left.

2: Remove the RF Modulator top screening plate

Screening Can RemovedRemove Screening Can

Next you will need to remove the top plate from the RF Modulator screening can. It simply prises off – a flat blade screwdriver will help you. Be careful not to slip and cut yourself though!

With the Screening can removed you will be able to see the RF modulator circuit inside. We are going to remove this circuit board completely in the next steps.

3: Disconnect the Modulator 5V supply & composite video feed

Remove Cables

The RF Modulator has two connection cables from the main PCB, as shown in the photo. One is a 5V power supply and the other is a composite video feed. These need to be de-soldered from the main PCB. The easiest way to do it is to heat the joint from the top side of the board and then, with soldering iron still applied to the joint, carefully pull the cable through the joint with a pair of pliers. Be careful when you do this as there is a danger of flicking up molten solder – unless you wear glasses then basic eye protection should be worn.

4: Disconnect UHF feed resistor from RCA Connector

Disconnect Resistor

Now you need to disconnect the feed resistor from the RCA connector shown in the photograph. You can either desolder it or just snip it off with a good pair of wire snips. The choice is yours! If you’re going to snip it off then make your cut close to the RCA connector itself – this way you can save the RF modulator circuit complete and it will be easy to reinstall in the future in the unlikely event that you decide to put it back.

5: Remove the RF module from the Main PCB

Desolder & remove the RF Module

Now it is time to remove the RF module completely. To do this you will first need to remove the Main PCB from the bottom casing. This is very easy to do – there is just one screw securing the Main PCB to the bottom casing so release it and the Main PCB will come away easily.
After this you need to desolder the RF module anchor points which are shown in the photograph. It can be a little tricky to desolder these because they are connected to a huge ground plane which – in conjunction with the screening can itself – tends to sink the heat of your iron away from the solder joint itself. If you have an adjustable temperature on your iron you will want to turn it up to full for this particular job. It is quite difficult to completely free the joints of solder so you will probably find that the removal process is a case of heating the joint and carefully prising the module free a little bit each side at a time.

6: Remove RF Modulator PCB

Remove the modulator PCB

With the RF module removed it is time to remove the modulator PCB. This is very easy – there are four anchor points as shown in the photograph. Desolder these and the PCB will come free. You will then be left with an empty screening can which you’ll want to solder back in to the board so that you can re-use the RCA cable.

7: Fit decoupling capacitor

Fit decoupling capacitor

With the empty screening can fitted back onto the PCB as shown in the photograph you’re ready to fit a decoupling capacitor between the machine’s composite video connection point and the RCA connector. All you need for this is a 100uF 10V capacitor. I used a 16V part because it’s all I had in my parts bin – it works fine. At these kinds of capacitances you’re probably going to be using a polarised part (most likely an electrolytic like the one I’ve used). In that case connect the positive end of the component to the RCA connector and the negative end to the main PCB. On an electrolytic capacitor like the one shown, the negative lead is designated by a marking on the component body itself.

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NOTE: The decoupling capacitor isn’t really that essential. It may improve picture quality for certain models of television set – it depends how the composite video input circuitry works – but on other models you probably wouldn’t notice any difference. If you don’t have a suitable component around you and you can’t be bothered to order one then you can just try connecting a cable straight from the composite video point on the Main PCB directly to the RCA connector itself. It should work.

8: Test

Time to test it! Don’t bother putting it all back together just yet – connect up the power and connect the RCA connector to your TV composite video input using a standard RCA cable. If all is well (and it should be) you will see a nice clean video output!

9: Enjoy!

I hope you enjoyed this simple modification. If you try this mod yourself then I welcome comments – please feel free to join the discussion! I will leave you with a video I made of the modification that I made it while I was still on a voyage of discovery with it myself – you may still find it useful.

10: Links

Here are some links to other good sources of information on modding your Spectrum:

VIDEO:

Happy New Year!

2012 was a tough year for me personally, and I only set myself one goal for the year which was to start (and continue) giving blood. I achieved that goal and have given blood 3 times so far with my 4th appointment in just a few days.

Here are my goals for 2013:

Learn to play the guitar

My Tanglewood Discovery electro-acoustic guitar

This all started when I literally found an acoustic guitar in my attic while I was clearing out a load of junk. Unfortunately it didn’t last very long; the bridge (this is the part that holds up the strings on the base of the guitar) snapped shortly after getting it down from the attic.
The bug had bitten though and so I’ve decided to try learning the guitar in 2013. I have bought myself this lovely electro-acoustic guitar from Bridgend Music Store:

So far I have managed to learn a few chords and toughen my finger-tips up a little bit. I am told that this is a very important part of learning the guitar because unless perseverance is shown at this stage then I’ll never be able to progress with the more interesting stuff (learning to play actual songs) later on and I’ll end up getting nowhere.

Here’s to 2013 and learning to play a musical instrument!

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Design and sell my first electronics product

Beagle Bone single board computer

It has been my ambition to start my own electronics design business for a little while. It’s a tough industry to get into because to make good money you need to have quite a bit of money behind you as a budget for development, parts, PCB ordering, etc. I don’t have any real money behind me, and I’m in a bit of a catch-22 situation because in order to get some money behind me I really need to be doing some extra work in addition to my full time job. But to do the extra work I need some money behind me!

For this reason I’ve decided to start small. I want to design some expansion boards for the Raspberry Pi and BeagleBone single board computers. These will be relatively low quantity products targetted at the electronics hobbyist market. I have no idea if it’ll make me any money, but it’ll be fun designing them and I have an end-application of my own in mind for the boards as well so the effort will not be wasted even if the project never gets off the ground in terms of selling them on the market.

Launch my HAB project

A photo from near space

Another goal I’ve had for a number of years is to design and build a working flight computer for a high altitude balloon and launch it. This has been done many times before by other enthusiasts, and I’d like to have a go at it too. This project ties in to my first electronic design products for the Raspberry Pi and Beagle Bone computers; the expansion boards I’m going to design for them will be used in the flight computer I am planning to build and launch.

If you can think of any cool science experiments you’d like to see conducted at near space altitudes please get in touch in the comments below with your suggestion!

Continue my 2012 Give Blood goal

Give Blood

I plan to continue to give blood in 2013 and I would encourage you to do it too. For years I considered giving blood but didn’t end up making the time to do it. And that’s an important point; you need to make the time, because unless you do, you’ll never end up doing it. Once you make your first appointment and get into the routine it becomes easier to stick to the plan, and giving blood has been a rewarding experience for me. Make it your New Year’s resolution!

Introduction

The digital multimeter is the most widely used test instrument in the electronics industry. It is the standard tool for electronics Technicians and it’s usually the first test/diagnosis tool that a newcomer to electronics will purchase.
Despite this, multimeter capabilities and especially the concepts of multimeter accuracy are often misunderstood or ignored. I have worked in the electronics trade for 14 years and it has been my experience that surprisingly few people actually understand (or care about) their multimeter specifications. In particular, I have discovered that a large number of Technicians and even Engineers are ‘blissfully’ ignorant of their instrument’s capabilities and the implications for the measurements they make.

If you don’t know and understand your instrument specifications, how can you choose the right tool for the job? And, more importantly, how will you know when you’re using the wrong tool for the job?!

Digital Multimeter Specifications Explained

Modern digital multimeter accuracy specifications are actually quite easy to understand once you become familiar with all the jargon. It is important that you fully understand what is meant by counts, digits, and the effects they have on instrument resolution and accuracy. In terms of resolution and accuracy, there is an important distinction to be made here as well – in my experience lots of people get them confused.
In this tutorial we’ll tackle counts and digits first, and this will allow us to very easily interpret the accuracy specifications afterwards.

Digits, Counts and Resolution

Resolution

When we talk about resolution we’re talking about the smallest possible change that the instrument can detect. This means we’re looking at the least significant digit. The resolution at any given time is the amount that a single count of the least significant digit is worth. So, for example, if the display is showing us ‘4.0005‘ volts, then one count of the least significant digit is worth 100µV (0.0001V). This means that the instrument’s resolution for that particular measurement is 100µV. The resolution will change depending on what range you select, but for the most accurate results you should always use the lowest possible range, which gives maximum resolution. I’ll show you why this is important for accuracy (accuracy is a different concept) later.

Counts

My Fluke 28II multimeter is a twenty-thousand count, 4½ digit instrument. This refers to my instrument’s resolution, but what does it mean? Well, the counts and digits are effectively two ways of saying the same thing, but both terminologies are in common use so it’s good to have a handle on both. I’ll tell you my personal preference and offer justification for it later. In this section let’s deal with the counts first.
To start with, it should be noted that the practical count figure is almost always one count less than the naming convention we use to refer to it. For example, in my case (for a Fluke 28II), the practical resolution of my instrument is 19,999 counts. That is what the instrument is actually capable of. However, when we refer to the counts by name we call this “twenty-thousand count”, and this is purely because a round number is easier to say! What we mean in practice is one less than that. The instrument specifications will usually quote you the practical counts as an actual figure, so with a well written specification there should be no ambiguity:

Fluke 28II Resolution Specifications

Displaying 1.9999V with 100uV Resolution

The implications in terms of multimeter resolution are that the Fluke 28II is capable of displaying a maximum of 19999 on its screen. A point to note here is that the most significant digit can ONLY be a 0 or a 1. It can of course move a decimal point to indicate different orders of magnitude. So if we’re measuring <2V, the instrument can display up to 1.9999V. What happens when we try to measure voltages higher than this? Well, the instrument has to abandon the most significant digit because it can’t display a ‘2’. This has the following consequences:
In the case of a 1.9999V measurement the least significant digit being displayed is worth 100µV per count (0.0001V), and therefore the instrument has 100µV resolution up to 1.9999V. Once we enter the 2V realm the instrument has to sacrifice some resolution because the most significant digit cannot display a ‘2’. Therefore in order to display 2V it has to shift the displayed measurement to the right, and the current least significant digit gets bumped off the end of the display in the process (i.e. we lose it).
The displayed voltage would be 2.000V, and the least

Displaying 2.000V with 1mV Resolution

significant digit is now worth 1mV per count. It’ll then maintain this 1mV resolution all the way up to 19.999V after which it’ll be forced to drop a least significant digit again and the resolution will become 10mV per count.
You can see, then, that once you know your instrument’s maximum number of counts you can use this information to determine what the maximum resolution will be for any measured voltage. The resolution will decrease in discrete steps as the measured voltage increases. The point that the steps occur and their effect on the resolution are determined by the maximum number of counts.

Digits

So how does all this relate in terms of digits? Very simple. The multimeter is a 4½ digit instrument because it is capable of displaying four full digits (0-9) plus one half digit. The most significant digit is called a half digit in this case because it is only capable of displaying 0 or 1.

4.5 Digit Multimeter Display

Some instruments are capable of displaying higher numbers in their most significant digit. Commonly you will see a ¾ digit quoted, and this usually refers to a digit that can display up to and including a numeric value of 3. So, for example, a 4¾ digit multimeter could display up to 39999 on its display. This would be called a “forty-thousand-count” instrument, and it is an improvement over the 19999 count display because it can go further into its range before it has to compromise its resolution by dropping a least significant digit.

There is a caveat here though – although a ¾ digit typically refers to a digit capable of displaying values between 0 and 3, this is not a safe assumption and in fact it can mean any digit up to 6. This means that there is some ambiguity surrounding the use of fractional digits to define resolution.

Counts And Digits Are Equivalent And Interchangeable

Counts and digits effectively mean the same thing. A twenty-thousand-count instrument is capable of displaying practical values of up to 19999 which is four full digits plus one half digit = 4½ digit.
Due to the uncertainty of meaning surrounding fractional (in particular ¾) digits, it is my opinion that the use of counts to define resolution is preferable because it accurately defines the instrument’s capabilities and leaves no room for ambiguity.

The Display is not the limiting factor!

Before I leave my explanation of multimeter counts, digits and resolution, I want to clear up a common misconception. Some might reasonably question why the instrument manufacturer would choose to hamper themselves with a most significant digit that can only display a 0 or a 1. Would it not be easier to have a full digit there as well, thereby avoiding the complications and maintaining better resolution for more of the range?
Well, the answer is that the display is not the limiting factor here. The display itself is almost certainly quite capable of indicating numerals from 0-9. The limiting factor is the measurement circuitry in the instrument itself . All instruments obviously have a finite resolution, and it is this limiting factor that causes the instrument manufacturer to be tied to a smaller MSD.

The Meterman 37XR, for example, has a ten-thousand-count display (actual counts 9999). The ten-thousand-counts refers to the resolution capabilities of the instrument itself (the lower the number, the less resolution the instrument provides), and in this case the consequence for the display is that it can indicate up to 9999V + decimal point. So in this case the most significant digit really can display 0-9, and there is no fractional digit there to complicate matters. But we only have 4 digits of displayable resolution across the range. We don’t have access to an extra ½ digit or ¾ digit at all, so we never get to exploit the extra resolution that this part-digit would provide. A part-digit that offers an order of magnitude better resolution for part of the measurement range, is better than no digit at all.

Multimeter Accuracy Specifications

Now that we fully understand the meaning behind counts, digits and resolution, we can quite easily interpret a digital multimeter’s accuracy specs.

What does ‘accuracy’ mean?

The accuracy of a measurement refers to how closely it reflects the true value of the property being measured. Whenever you measure something in real life, the measurement you take is always an approximation of the actual property itself, and therefore there’ll be some uncertainty involved. Today’s digital multimeters are very accurate instruments – the uncertainty in their measurements is extremely low – but there will always be some uncertainty in the measurement.
What will the error be? Well, it’s impossible to quantify the error exactly. If you think about it, if we could determine the exact magnitude of the measurement error then we’d just correct for it in software and then we’d have no error at all! That’s why we refer to it as “uncertainty” instead of “error”.
In practice all we can really do is provide a figure of uncertainty about the measurement which gives us a range for which the measurement can potentially be in error. The multimeter specifications give us these limits, and they’re called the accuracy specifications.

So we have dispelled the jargon, and this makes our life easy. Let’s now look at some practical accuracy specifications and determine what they mean. Staying with the Fluke 28II, let’s have a look at its accuracy specifications for the VDC range:

Fluke 28II DC Specifications

As you can see, the Fluke 28II’s DC voltage range is quoted as being accurate to “±0.05% of the reading +1”. The ‘+1’ refers to an additional uncertainty in terms of ‘numbers of counts’. Some manufacturers refer to this uncertainty as ‘numbers of digits’, but they both mean exactly the same thing – it’s basically the number of counts in the least significant digit. In this case we’re only talking about one count of uncertainty but some instruments suffer more than that. I prefer the former terminology (counts) because it sounds less confusing! Notice that the +1 count is contained within the ± bracket so the actual uncertainty in terms of counts is plus or minus 1 count. The easiest way to understand what this means in terms of measurement uncertainty is to take an example.

Example: Measurement uncertainty for a known 1.8000V source with the Fluke 28II.

Let’s imagine we decide to measure a voltage reference whose true voltage is known to be 1.8000V. If we measure this with the Fluke 28II using the most appropriate range (more on this later!) we can expect that the instrument’s measurement uncertainty will be:

This means we should expect a measurement of somewhere between 1.7991V and 1.8009V. However, this isn’t all of the uncertainty we can expect to see on the display because we also have an additional uncertainty (which is due to ADC errors, offsets, noise etc) of ±1 count, and this gets added on to the least significant digit being displayed. So, adding that to the measurement uncertainty we get 1.7990V to 1.8010V. We should expect to see a measurement on the display that is somewhere between these two values. Easy! Let’s have a look at what this means for an instrument with slightly lower resolution and accuracy specifications:

Example: Measurement error for a known 1.8000V source with the Meterman X37R

Let’s try this same task with the Meterman X37R. The specifications for the VDC range are:

ACCURACY: ±(0.1% Reading + 5 digits)

RESOLUTION: It’s a 4 digit instrument (no partial digits) which is 9999 count so our maximum resolution when the most appropriate range is used for this particular measurement will be 0.001V = 1mV.

Using all this information, the uncertainty in our measurement will be:

This means we should expect a measurement somewhere between 1.798V to 1.802V. But then we have the additional uncertainty of 5 counts on top. Not only is there a greater uncertainty of counts to add in this case but now they’re more meaningful too because the least significant digit is more significant than it was for the same measurement with the Fluke 28II – the 37XR has less resolution. The 5 counts get added to the 1mV column, where as the Fluke’s ±1 count uncertainty only got added to the 100μV column!
This gives us an overall expectation of a displayed reading on the 37XR of somewhere between 1.793V to 1.807V. You can see how an instrument with lower accuracy and lower resolution can start to make a difference.

Always use the most appropriate range!

There’s a consequence for all this here that we haven’t talked about, and it refers mainly to the count (or digit) errors quoted in the specifications. You must always use the most appropriate (highest resolution) range for the property being measured. If you don’t, the resulting measurement errors can end up being quite large because the count uncertainties carry more weight. Let’s say we do the same experiment with the 37XR, but this time we use the 1000V range to take the measurement. The displayed measurement will then be somewhere around 1.8V – we’ll be wasting the other two digits that are set to take tens and hundreds units, because there are no tens or hundreds to measure! We’ll still end up with the same measurement error in this case (it’s still ±0.1% Reading + 5 digits), but the 0.1% uncertainty is too small to be registered on such a low resolution display. The counts, however, do register because they always affect the least significant digit being displayed – which in this case is the 100mV digit (the 8). So the actual reading displayed could be between 1.3V and 2.3V! That’s a total error of ±28%which, as I’m sure you’ll agree, is completely unacceptable. So watch out for that and always make sure you make use of the best possible measurement range!

That’s all folks!

So there you have it, digital multimeter specifications explained. It’s really quite simple once you get down to it. The topic is a little bit more hard work for analogue instruments – I’ll tackle that little minefield in a separate tutorial.

Gotcha!

I’ve experienced a problem recently whilst measuring high voltages with a 1000:1 high voltage probe and an Agilent (U1253B)/Fluke (28II) multimeter. The problem is that the two meters don’t agree with each other!
The Fluke (my meter of choice) returns voltage readings within expectation all the way up to 12kV (12V as displayed on the instrument). But the Agilent meter only seems to agree with the Fluke up to about 3kV, after which it starts to drop off. By the time we get to 12kV the Agilent is reporting a voltage that is more than 1000V less than expectation. I tried another Agilent U1253B and experienced the same drop off. What is going on here?

Know your input impedance

So I started to think about input impedance. The high voltage probe is designed to work with a 10MΩ impedance. Both these high-spec handhelds will be 10MΩ, right? That’s standard for handhelds these days. Is this a fair assumption?
It turns out, no it isn’t!!!

Firstly, RTFM. Both the Agilent and Fluke claim 10MΩ input impedance for the D.C. voltage range in their manuals. However, the Agilent has a fancy dual display mode whereby you can measure two different properties (say, A.C. and D.C. voltage) simultaneously. In this mode each display presents as a 10MΩ impedance so you end up with an effective impedance of 5MΩ in total.

…but I wasn’t using the dual display mode, so I should expect 10MΩ, right? Well, that’s what the manual says. But let’s measure it!

Measure the Agilent’s Single Display Input Impedance Using the Fluke

Firstly we connect the Fluke up to the Agilent and take a resistance measurement of its inputs. We should expect to see 10MΩ, and sure enough we do:

Measuring the Agilent’s single display input impedance using the Fluke.

Measure the Agilent’s Dual Display Input Impedance Using the Fluke

Next we set the Agilent to dual display mode and take the measurement again. We should see 5MΩ, right? Yes! So far so good…

Measuring the Agilent’s dual display input impedance using the Fluke.

So far we seem to be doing well. The Agilent’s input impedance is as expected.

But there’s a problem…

There’s more than one way to measure the Agilent’s input impedance. We can use an insulation tester. The difference is that the Fluke is applying a constant current and then using the measured voltage drop to calculate resistance, where as an insulation tester does it the other way around – it applies a constant voltage and, I presume, uses a measured current to calculate the resistance. It’s six of one and half a dozen of the other – both types of measurement should agree with each other. But do they? Let’s find out:

Measure the Agilent’s Input Impedance Using the Insulation Tester

Here we connect the Agilent up to the Insulation Tester. I tried it at various test voltages and they all agreed with each other, but there’s a surprise in store – the insulation tester reports 5MΩ input impedance for the Agilent’s voltage measurement range. And this measurement is reported regardless of whether the single or dual display mode is used!

Measuring the Agilent’s Single Display Input Impedance Using the Insulation Tester

What is going on here? Why is the insulation tester reporting 5MΩ input impedance for the Agilent’s single display mode? And could this explain my measurement problems with the high voltage probe? I think it could! But in that case, what can we say about the Fluke’s input impedance? Let’s measure it, first with the Agilent and then with the insulation tester:

Measure the Fluke’s Voltage Range Input Impedance Using the Agilent

Okay so we connect the Fluke up to the Agilent and measure its input impedance using the Agilent’s resistance range. We get 10MΩ as expected:

Now we measure the Fluke’s input impedance using the insulation tester. We should get 10MΩ:

Indeed we do get 10MΩ. So, to summarise:

The Agilent’s single display input impedance measures 10MΩ using the Fluke’s resistance measurement, but the insulation tester says it’s only 5MΩ – and this is regardless of the display mode – both the single and dual modes look like 5MΩ to the insulation tester.

The Fluke, on the other hand, looks like a 10MΩ impedance to both the Agilent multimeter and the insulation tester. This is what you would expect.

I tried the measurements again with another Agilent U1253B and I experienced the same thing. I also experienced the same voltage measurement problems when using the high voltage probe. This rules out a faulty instrument.

So what is going on?

This is a very good question! Why does the Agilent look like a 5MΩ impedance to the insulation tester? Why is it not 10MΩ as stated in the manual? And why is there a discrepancy between the insulation tester measurement and the multimeter measurement? This discrepancy isn’t seen when we measure the Fluke.

This input impedance problem provides an explanation for the voltage measurement errors I’ve experienced. The high voltage probe I’m using is designed to work with a 10MΩ multimeter, so a lower impedance instrument is going to present a problem. This is what I’ve experienced in practice. The Fluke, on the other hand, works with the high voltage probe no problems at all.

Misleading Impedance Specifications

Here’s a copy of the input impedance specifications from the Agilent U1253B user manual:

As you can see, they are quoting 10MΩ for each VDC measurement range, from 5V to 1000V. However, there’s a problem with this! Refer to note 3 in the fine print below the table. That’s right – the input impedance actually varies with input voltage! So, even though they quote 10MΩ input impedance, it’s actually only 10MΩ for input voltages between -2V and +3V! Outside of that it’s only 5MΩ.

To put that in perspective, -2V to +3V is less than 0.3% of the instrument’s total range. So for 99.7% of its range, the impedance is only 5MΩ. Despite this, they somehow think it’s informative to quote the input impedance as being 10MΩ. That’s a bit bizarre.

Anyway, this fact explains why the insulation tester and the multimeter disagreed over the input impedance. The multimeter’s constant current stimuli yields a voltage that is <1.5V so it comes in on the 10MΩ impedance zone. The insulation tester’s minimum voltage stimuli of +50V is well into the 5MΩ impedance zone.
Also, the fact that the instrument has 5MΩ impedance above +3V explains why it starts disagreeing with my Fluke after about 3kV. The high voltage probe is designed to work with a 10MΩ impedance so as soon as the Agilent’s impedance changes over to 5MΩ, erronous measurements are returned.

The Moral of the Story Is:

Never assume your instrument’s input impedance! It’s not necessarily 10MΩ! And, in the case of Agilent, even if the manual quotes 10MΩ make sure you read the fine print because the might have been misleading you!

Anyone who has ever worked in the electronics trade will almost certainly have been asked to repair consumer electronics products for friends, family and even random neighbours. How do you deal with these requests? Do you politely decline or do you end up getting sucked in?
Rookies usually get sucked in. I’ve been there, done that, and got the T-Shirt. But give yourself a few years and you’ll soon learn that it can be a huge “trap for young players” (as EEVBLOG is fond of saying), and that once you’ve fallen into the trap it can be very difficult to get out!

I’m older and wiser now, so usually I’ll politely decline a request for this sort of work. Occasionally I’ll agree to do the odd thing for close friends and family, but even then I only tend to agree if I feel confident that the symptom is indicative of a quick/easy and permanent solution. If there’s any kind of uncertainty involved, or if it’s someone I don’t know, forget it – I’ll avoid it like the plague. Why? Well, let’s have a look at it!

My main reasons for declining this sort of work are as follows:

Once you agree to repair something for someone, suddenly everyone in the neighbourhood will want you to provide a similar service for them too – and once you’ve agreed to do it for one person, it becomes difficult to say no to anyone else! How can you justify saying no to Mr. Jones at number 4 when you previously said yes to Mr. Edmunds at number 3? You’re almost obligated to become the local repair guy, and from then onward your spare time will be constantly eroded by other people’s problems.
At least you can make a few bob for yourself on the side though, right? No! That neatly leads me on to my next gripe…
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Nobody ever wants to pay you money for the work. They think your technical expertise in this area is worthless, and in one way (as explained further on in this point) they are right!
It doesn’t matter that you may have spent 4 hours tracking down a problem, and that the only reason you can do it at all is because you spent years (decades) honing your electronics skills – they’ll still want it done for free. Or, at least, for a very small amount of money.
In fairness, this kind of attitude has mainly been fostered by cheap consumer electronics products from countries like China. Your diagnosis/repair work might be worth £80 an hour in terms of your expertise, but why are they going to pay that when they can just get a brand new one from the local supermarket for <£100? Cheap goods from developing countries have literally decimated the monetary value of a Technician’s work. Products have become more complex and hence more difficult to diagnose, but the amount that consumers are willing to pay for their repair has fallen to a pittance.
Naturally Mr. Jones won’t want to add £100 to his next TESCO shopping bill though – he’ll just want you to fix the one he’s got for free.
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Once you’ve placed your hands on someone’s product, you instantly inherit any future problems it may present. If Mr. Jones bought you a pint of beer in return for restoring power to his television last month, then he’ll bring it back to you when the colour goes down on it and assume that the two problems are linked. “The colour was fine before you started fiddling with it”, he’ll say. “It must have been something you did!”.
Naturally, he’ll not only expect you to fix his colour problem as well but he’ll also expect you to do it for free.
Sometimes you’ll even be blamed for totally unrelated things like, for example, poor reception. You fix their dead television for a measly tenner (even though it’s not even worth 15 minutes of your time, and the job took you three hours!) but then they’ll call you back 3 months later because the picture on ITV4 is breaking up. “It didn’t do that before you took the TV apart, Mr. Hoskins!” and before you know it you’ll be up on their roof fixing an antenna or adjusting their satellite dish.
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Sometimes, if you don’t have your wits about you, amateur diagnosis work (and if it’s done in your spare time as an aside to a related but different professional trade, it is amateur – even if your skills are not) can even end up costing you money. Faults (especially in the digital domain) can be very difficult to diagnose, and are littered with “gotchas” that you can inadvertently stumble into. Fault symptoms will often lead you around the garden path.
It can seem, for example, like a Microprocessor is to blame for your problem when in fact the firmware is simply hanging up because some other component is upsetting it. But if you go ahead and order a replacement Microprocessor you’d better be damned sure it’s going to solve the problem, because Mr. Jones isn’t going to want to pay for it if it later transpires that the Micro wasn’t the root cause after all!
If you work in the diagnosis trade (which, these days, hardly anyone does) problems like these are easily navigated – you can swap components on like products to see if the problem moves with them, and then you can be more confident of your diagnosis before you spend any money (even though at the very least it’ll certainly cost you more unpaid time), but if, for example, you’re a design Engineer who (by the very nature of your work) happens to also possess some skills from the fault diagnosis trade, you won’t have this luxury. When Mr. Jones presents his faulty product to you it’ll almost certainly be the first time you’ve ever seen one. If you’re lucky (provided Mr. Jones never brings it back) it’ll be the last time you ever see one! So the moral of this particular point is that if you order parts for someone’s faulty product, make sure you’ve made your disclaimer clear before you do so otherwise the cost could be coming out of your pocket, not theirs!
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The final reason I prefer to decline this sort of work is that it just isn’t my specialty. Yes, I probably could track down the fault on someone’s TV, or laptop, or PVR. Given enough time and sufficient motivation I could probably fix any electronic product. But unless you actually work in the trade, diagnosing these things day-in day-out, it’s always going to cost you more time and money than it’s actually worth. For a start you’ll hardly ever have schematics for the products, and that means you’ll have to reverse engineer them before you even start diagnosing the problem. You won’t have spare parts hanging around so you won’t be able to follow hunches by swapping bits out, and finally you’ll never have the opportunity to reap the rewards of a hard-earned diagnosis. What do I mean by this last point? Well, no Technician wants to see a one-time fault. Obscure one-time faults do happen occasionally, but usually (if you work in the trade) it’ll be something you or one of your colleagues have seen before. So you invest the time in a diagnosis once, and then you apply it instantly to any future occurrence of the problem. In that way, you start to make money on your investment. If a product costs you six hours of diagnosis time, most of which will end up being unbilled time, then you hope that it’ll pay you back when you see the problem again in the future.
When you do these things as a side job, you typically only ever see the problem once, even if it’s a relatively common problem for that particular product. So each time you complete a diagnosis you invest significant time, but never see its return. Even if people were still willing to pay good money for diagnosis work (which they’re not), it would hardly be worth it for someone who just does odd bits on the side.

No I will not fix your computer!

So that’s why I will almost certainly decline any request to fix someone’s consumer electronics product for them.
What about you? Are you a design/development engineer who has been asked to fix other people’s stuff? Do you agree to it or do you decline? What stories can you tell?!

So, picture the scene; you’ve been up since 6am, you’ve suffered the dreary commute into work, you’ve slaved away all morning without stop, and you’re looking forward to sitting down for five minutes peace with a nice bite to eat. You’ve brought along a HEINZ beef in peppercorn sauce ready-meal, and it sounds delicious. A nice hot meal cures all evils, doesn’t it? You’ll wolf it down, have a nice cup of tea, and be fighting-fit for the afternoon session.

You pick up your ready-meal, ready to go in the microwave, and look at the front:

mmmmmm, yes I most certainly will enjoy! Lovely!

Imagine your disappointment, then, when you find that your piping hot meal is rather less than as described? “Beef in Peppercorn Sauce”, it said. “…with potatoes and a mix of sweetcorn and peppers”. So, in my mind, I am expecting the following:

Beef in the majority, then slightly less Peppercorn Sauce, then slightly less potatoes, and then finally sweetcorn and peppers in the minority.

Is this an unfair expectation? A presumption? I think not! That’s how I rationalise their description in my mind, and I’m sure it’s how you do it too. But you sit down with their meal and find that actually, you’ve just eaten a potato & vegetable dinner with peppercorn sauce. Was there even any beef at all? If there was, it was lost in the melee of potato and vegetables!

This, is what my Grandfather would have called, a “swizz“. And there’s nothing quite like a lunch-time swizz to darken your mood for the rest of the day; a plight to which I’m sure most people can relate. Ever opened up a packet of crisps only to find 7 crisps inside and the majority of the packet a wide open space? That’s what we’re talking about here. A swizz!

So, let’s get to the bottom of this then shall we? Exactly how much beef is in a “Beef in Peppercorn Sauce” ready meal from HEINZ? I happened to have another of these meals in the freezer at home so I decided to find out. Here’s what I did:

First, let’s take a look at the opened meal:

It looks rather delicious I’m sure you’ll agree. But suspicious as well! Where is all the beef? I think I can just about see one lonely piece sticking out at the bottom right corner there. But that’s it. No more beef to be seen anywhere. Well… perhaps all the beef is on the bottom?

Alright then, so let’s microwave it and separate everything out:

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Potatoes

So, here’s the separated out potatoes. That’s quite a lot!

Since most of them were on the top I managed to separate them out quite cleanly. There were a couple at the bottom that have a little bit of sauce on them, but small enough to be negligible. I weighed this little lot in at 125 grams.

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Sweetcorn & Peppers

Next up we have the sweetcorn & peppers. These were very easy to separate as they were in a completely separated compartment of their own. This little lot weighed in at 100 grams.

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Peppercorn Sauce

Separating the peppercorn sauce was a little bit more tricky, mainly because of the beef – we’ll get onto that in just a moment. The Peppercorn sauce consists of the creamy sauce itself, a couple of mushroom slices, and some other bits and pieces that I wasn’t able to make out, but which were certainly not beef. I did a pretty good job of separating the sauce from the rest of the meal, and I got the vast majority of it out. This lot weighed in at 135 grams.

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The Beef!

Finally, we get onto the beef! The beef was the most difficult of all to separate, mainly because there was hardly any of it to fish out! After I did get it all out, a lot of it had a significant amount of sauce on it and I felt that this would manifest itself as an unacceptable error in the weight measurement. So, what to do? Well – quite simple really! We weigh it as it is with all the sauce still on it, then we wash it and weight it again. All the sauce will have been washed away and we’ll be left with the beef itself. The second measurement is the amount of actual beef in the ready-meal, and this measurement subtracted from the first gives us the amount of sauce that was washed off, which we can add to the peppercorn sauce total. I already accounted for this in my quoted peppercorn sauce measurement so the 135 grams quoted previously is the total amount of peppercorn sauce in the meal.
How much beef is there? A measly 25 grams!!!

PERCENTAGES

So how does all this work out in terms of percentages? Well, we have:

25 grams of beef

135 grams of peppercorn sauce

125 grams of potatoes

100 grams of vegetables (sweetcorn & peppers)

That little lot all told gives us 385 grams worth of meal. So the percentages, broken down and listed in descending order, are:

Peppercorn Sauce 35%

Potatoes 32.5%

Vegetables 26%

Beef 6.5%

So, by my reckoning, the meal should actually be titled:

Creamy Peppercorn Sauce and Potato Chunks.
…with sweetcorn and peppers and small traces of beef.

So thank you for that HEINZ. I’m fortunate enough that I don’t need to lose any weight, but if I ever do your weight-watcher meals will be just the ticket! Diet by lack of content – fantastic idea!

In closing, I think I should be fair to HEINZ and admit that the meal was actually rather tasty – I enjoyed it! All they need to do is add a lot more beef in it, or otherwise change its title. By the way, I love the “even more” quote underneath the package title! Even more what? More potato? More sauce? More vegetables? Surely not more beef?!